1. Field of the Invention
The present invention relates to an image forming apparatus and a method for controlling density of an image.
2. Discussion of the Background Art
In an image forming apparatus, such as a copier, a laser printer, etc., using an electrophotographic system, the following control is executed so as to always obtain constant density of an image. Specifically, a gradation sequence pattern including ten to seventeen toner patches for density detection use are formed based on a different image formation condition (i.e., a development potential) so as to attract a different amount of toner on a latent image carrier, such as a photoconductive member, etc. Based on a detection value of the toner patches obtained by an optical sensor, a toner attraction amount is calculated per toner patch using a prescribed algorithm. As a result, a linear equation formula (y=ax+b) is obtained from a relation between the toner attraction amount and the image formation condition of the respective toner patches. Then, development gamma representing a developing ability (i.e., an inclination “a” of a linear equation formula in a coordinate in which development potentials and toner attraction amounts are plotted on lateral and vertical axes, respectively), and a development start voltage Vk (an intercept “b”) of the linear equation formula are obtained. Then, an image formation condition, such as a LD power, a charge bias, a development bias, etc., is adjusted to control a prescribed development potential attracting an appropriate amount of toner based on the development gamma and the development start voltage Vk.
The optical sensor detecting the toner patches includes a light emitting element such as an LED and a light-receiving element such as a phototransistor, and detects a reflection light from the light emission element by the light-receiving element. The optical sensor is generally sensitive in detecting a small amount of toner attraction toner, while not sensitive in detecting a large amount thereof owing to detection sensitivity of the light-receiving element. Specifically, the optical sensor includes a prescribed sensitive range capable of sensitively detecting a toner patch. Thus, in order to precisely obtain a development gamma and a development start voltage Vk, an amount of toner attracting to each of plural toner patches constituting a gradation sequence pattern needs to disperse at the same interval from small to large toner attraction sides to be sensitively detected by the optical sensor.
In the past, ten to seventeen toner patches each attracting a different amount of toner are formed based on differently fixed development biases, respectively, so that the development gamma and the development start voltage Vk can be precisely calculated regardless that the development gamma is either high or low. As a result, when the development gamma is high (sharp), the developing ability increases and an image is formed attracting a lot of toner even formed based on a low development bias. Further, an amount of toner attracting to a toner patch even formed based on a medium development bias sometimes deviates from a detectable range for the optical sensor. Thus, to disperse from small to large toner attraction amount sides at the same interval within the detectable range for the optical sensor, plural toner patches of a gradation sequence pattern need to be formed based on the low development bias when the development gamma is high. Whereas when the development gamma is low (dull), the developing ability decreases and an image attracting a lot of toner cannot be formed unless the development bias is high. When the development gamma is low and toner patches are formed based on a low development bias, even the highest toner attraction amount of the toner in the gradation sequence pattern becomes relatively low, so that the toner patches concentrate on the low toner attraction amount side as a whole. When concentrated on the low toner attraction amount side, the development gamma and the development start voltage Vk cannot precisely be calculated due to affection of unevenness of the toner attraction amount. Thus, in order to precisely calculate the development gamma and the development start voltage Vk when the development gamma is low, toner patches formed based on a high development bias are needed beside those formed based on a low development bias.
Thus, to precisely calculate the development gamma regardless of its level, plural toner patches formed based on high and low development biases are needed. As a result, a conventional gradation sequence pattern needs ten to seventeen toner patches. However, when a lot of toner patches are used, adjustment of image density takes a longer time period increasingly consuming toner.
Japanese Patent Application Laid Open No. 2006-106222 describes an image forming apparatus capable of controlling density of an image. Specifically, a development gamma and a development start voltage Vk obtained based on a result of detection of a gradation sequence pattern by means of an optical sensor are stored in a memory. When the next image density control is executed, development biases for forming respective toner patches are calculated based on the development gamma and the development start voltage Vk stored in the memory so that an amount of toner attracting to each of the toner patches disperse from small to large toner attraction amount sides at the same interval within a detectable range for a sensor. Then, the gradation sequence pattern is formed based on the thus calculated development biases and controls image density. Generally, the development gamma does not significantly vary from the previous value.
Thus, as far as the respective toner patches of the gradation sequence pattern are formed based on the development gamma obtained last time so that an toner attraction amount of each of the respective toner patches disperse from small to large toner attraction amount sides at the same interval within the detectable range for the optical sensor, a number of the respective toner patches can be decreased. Specifically, when the development gamma is high, all of the toner patches in the gradation sequence pattern are formed based on the low developing bias, while the development gamma is low, the toner patches on the low toner attraction amount side are formed based on the low developing bias and those on the high toner attraction side are formed under the high developing bias. Thus, by changing the developing bias for the toner patches based on the development gamma calculated last time, a small number of toner patches are enough if dispersing from small to large toner attraction amount sides at the same interval within a detectable range for the sensor.
As a result, due to decrease in number of the toner patches, image density control can be quicker while toner consumption for image density control can be reduced.
However, when environment sharply changes from the last time or image density control is executed after long time absence of an apparatus or the like, development gamma sometime significantly increases.
When the development gamma significantly increases from previous one, only a toner patch on the low toner attraction amount side among those formed based on a development bias calculated based on the development gamma of the last time enters a detectable range for the optical sensor, and thus the development gamma cannot sometime be calculated in an apparatus, in which a number of toner patches of a gradation sequence pattern is decreased by forming the toner patches based on the development gamma calculated last time. In such a situation, the image forming apparatus of the Japanese Patent Application Laid Open No. 2006-106222 forms a gradation sequence pattern again by changing a developing bias or increasing the number of toner patches so that data of an toner attraction amount of at least two toner patches can enter the detectable range for the optical sensor.
However, repetitious formation of the gradation sequence pattern necessitates a longer image density control time period resulting in a long downtime of an apparatus.
In addition, consumption of toner for image density control use increases.
Then, the applicant is developing a new image forming apparatus. Specifically, the below described image forming apparatus is in the course of development, which forms respective toner patches based on a prescribed fixed developing bias, which always causes the respective toner patches to enter the detectable range for an optical sensor even if the development gamma is predicted to widely change. Further, when there actually exists a contribution factor, which significantly increases the development gamma this time more than that calculated last time and the development gamma significantly increases more than that of last time due to sharp environment change, the toner patches can disperse at the same interval within the detectable range for the optical sensor by fixing image formation conditions for all of forming toner patches.
There exist various contribution factors possibly increasing development gamma this time more than that calculated last time. Thus, detection of all of the contribution factors increases cost of an apparatus. Thus, only a principle contribution factor, such as environment, an absent time of an apparatus, etc., is detected. However, development gamma sometimes increases more than that calculated last time due to a contribution factor other than the principal contribution factor. For example, even though the principle contribution factor is not detected, and it is determined that development gamma this time does not increase than that of the last time thereby a gradation sequence pattern is formed based on a developing bias calculated last time, the gamma sometimes significantly increase this time than that calculated last time due to a contribution factor other than the principal contribution factor.
In such a situation, only one point of a toner patch on the low toner attraction amount side can possibly enter the detectable range for the optical sensor.
As a result, the development gamma cannot possibly be calculated.